It is expected that various types of services are to be provided by means of different Application Servers (AS) over IMS systems. An example of the AS service is the “Push to talk over Cellular” (PoC) service, also known as PTT, Push-To-Talk service.
The service Push to talk over Cellular (PoC) is a direct voice service over the General Packet Radio Service (GPRS). The technology uses the capabilities of the IP Multimedia Subsystem (IMS) as specified by the 3rd Generation Partnership Project (3GPP) for enabling IP connections between mobile phones. The push-to-talk service is a two-way form of communication that allows users to immediately engage in a communication with one or more receivers, similar to a Walkie Talkie, simply by pushing a button on their handsets.
On the other hand, Push to talk over Cellular (PoC) calls are a one-way communication: while one person speaks, the other(s) listen(s). The turn to speak is requested e.g. by pressing the special “PTT” or “tangent” key and granted on a first come first served basis. Users can join the group session they wish to talk to and then press the tangent key to start talking.
In order to be able to communicate in a group session, a user needs to be an active member of that group session. A user becomes an active member of a group session by joining the group session. The user can be joined to several group sessions at a time. The amount of simultaneous group attachments is configurable by the operator and their maximum number may also vary according to the Push to talk over Cellular (PoC) server vendor from a couple to even twenty. It is intended that each Push to talk over Cellular (PoC) group attachment has to establish an own Session Initiation Protocol session (hereinafter referred to as SIP session) on the control plane.
In release 5 of the 3GPP (and forward), the principle is that the controlling SIP sessions are used to bind to the user plane connection in order to provide e.g. charging correlation information. In release 5 of the 3GPP (and forward) the binding is done by the Go-interface. As a result, there should be as many PDP contexts in the packet core as there are SIP sessions on the control plane, if the bearer authorization needs to be done. It is not possible to re-use an existing PDP context, except when a “best-effort” PDP context is used and the authorization is not required.
Since the control plane and user plane connection binding (the support of Go interface) is not defined in the packet core environment of the 2nd generation, and thus the binding of SIP sessions on the control plane with the bearer carrying the media flows on the user plane is not applicable, the Push to talk over Cellular (PoC) User Equipment (UE) may re-use the same PDP context at its own discretion.
However, after a 3GPP release 5 environment, the problem is completely new where the binding mechanism (Go-interface) offers the means for bearer authorization, but it is stated that media flows of different SIP sessions cannot be multiplexed in the same PDP context, since the release 5 does not define any solution to this problem. The Go-interface is, however, optional to deploy in the network. Then, it is always possible to let the bearer without authorization, and re-use the “best-effort” PDP context.
Of course, it would be possible to use just one controlling SIP session for all Push to talk over Cellular (PoC) group calls and one-to-one sessions and just modify its media parameters when needed (target ports accordingly to connections etc.), but it would cause problems, for example, when only one group attachment is possible, and the existing group call is forced to release when the user joins a new group.
The Push to talk over Cellular (PoC) User Equipment (UE) and the packet core of the network cannot currently allocate and maintain many (e.g. ten or fifteen) simultaneous Packet Data Protocol contexts (hereinafter referred to as PDP contexts). According to present knowledge, the first industry standard compliant Push to talk over Cellular (PoC) User Equipment (UE) to come shall support only one PDP context of streaming or interactive class. Further, the packet core supports only a couple (currently up to six or eight) of active secondary PDP contexts for each User Equipment (UE). Thus, maintaining many real-time PDP contexts for every Push to talk over Cellular (PoC) subscriber at the same time would bind a lot of packet core and radio resources. Still, the Push to talk over Cellular (PoC) user could hear only one media from one session at a time. On the other hand, several Push to talk over Cellular (PoC) group attachments are required to provide adequate Push to talk over Cellular (PoC) services.